Get it before you travel!
All eligible international travellers who wish to travel to the United States under the Visa Waiver Program must apply for authorization with an an official ESTA application.
Customs and Border Protection recommends that you apply for ESTA at the time you book your travel, but no less than 72 hours prior to boarding. Instant approvals are no longer available.
Travellers must have an ESTA even if they're transiting in the U.S.
Please ensure your travellers are aware.